1. Field of the Invention
The present invention relates to optimum color density and gamma curve determination and, more particularly, to a method and apparatus for determining the desired output characteristics of a color output device.
2. Related Art
In digital color output devices (e.g., monitors, ink jet printers, color presses, etc.), a color is described by a set of numbers that, when received by the output device from a host computer, will cause the output device to output a desired color. For example, in a color printing device the primary colors Cyan, Magenta and Yellow are combined in predefined combinations and amounts in order to make other colors. A predetermined set of numbers indicate to the printing device how much of each color is to be used to achieve a desired color.
However, in these types of color printers, the particular color printed depends not only on the set of numbers received from the host computer, but also on other factors such as, for example, the output characteristics of the printer itself. These output characteristics include variations in the purity of the colorants used, the order in which the colorants are applied, variations in the transparent properties of the colorants used and the characteristics of the substrate on which the colorants are applied just to name a few. Accordingly, there will generally be variations in the colors output from printer to printer or monitor to monitor, for example, when a static set of numbers are input to each device from a host computer. That is, a given set of numbers for Red, Green and Blue values will most likely produce a different color response on different color output devices (e.g., different monitors). A given CMYK value will most likely produce, for example, a different color on different CMYK printers. For this reason, RGB and CMYK color spaces are both said to be device dependent. The color output produced will depend at least to some extent on the particular output device used.
Because of the device dependence of the color space, the process of one output device (e.g., a printer) emulating the output of another output device is complicated. The term "emulation" is often defined, for example, as one output device (e.g., printer) having the same output or print characteristics as another device. These characteristics can be defined in subjective and objective terms. Subjective characteristics include terms of description such as "midtone punch", "flat", "vibrant" and "good shadow detail", for example. Objective characteristics that can be measured and defined are most commonly based on a standard which describes a set of objective standards of color print characteristics such as the "Standards for Web Offset Printing" (SWOP) publication. This objective set of standards uses terms such as "print contrast", "dot gain", and "gray balance", for example, to define print characteristics. If measured values of the output device fall within specified tolerances, the output device is said to be printing to SWOP specifications. Although SWOP does not necessarily define how a particular color will print, it does define generally how a particular printing device will operate. From this, an experienced printer operator will know roughly what a given CMYK value will produce if the printer or press is calibrated to SWOP. For purposes of discussion in the present application only, SWOP is defined as a device independent color printing standard.
There also exists a color matching system such as PANTONE, which consists of a series of reference inks. Standard methods exist for calculating PANTONE mixtures that simulate printer inks. Although a PANTONE color matching system does not use the same printing inks (C,M,Y,K) that an offset printing press would use, for example, it does specify the CMYK values that the PANTONE standard has determined are the closest match to a given PANTONE color. The published CMYK values of PANTONE can be loosely defined as yet another device independent standard that a CMYK device would try to emulate.
Presently, in order for an operator to calibrate a printer to a particular standard such as PANTONE, for example, the operator will input the CMYK values corresponding to the particular desired PANTONE color. The operator will then examine an output printed using those given values and, based on experience, vary one or more of the color values or calibration curves on a trial and error basis until a selected combination results in an output which sufficiently matches the desired PANTONE color. After the proper color values or calibration curves are determined by this trial and error process, the system can be recalibrated using those values to achieve the desired output. However, such trial and error processes are extremely tedious, time consuming and inefficient.
On the other hand, an operator may not always desire that the printer emulate another printer or standard. In this case, the response or output of the printer can be determined and set, for example, based on the manufacturer, customer or market demand. However, the operator must still perform a tedious trial and error process to derive the color values so that the system can be calibrated to achieve the desired output result.
Typical methods presently being used may attempt to achieve device independent color using an intermediate, independent color space such as CIELAB, or CIExyz between device dependent color spaces (e.g., between an RGB input device and a CMYK output device or between a CMYK device and another CMYK device). That is, a color input device such as a monitor which uses a device dependent color space such as RGB will go to a device independent color space such as CIE and then to another device dependent color space such as CMYK for output by a printer, for example. However, this process involves the use of complex transformation algorithms and procedures and the creation of such is technically out of reach of the typical everyday printer user.
U.S. Pat. No. 5,537,516 relates to a method of calibrating a color printer using a scanner for color measurements. A set of calibration curves are generated for correcting the color output of the color reproduction device. The calibration curves provide modification functions for each of the individual color print channels of the color reproduction device and are used to alter the rendering of color reproductions. However, this method requires a rather complicated and lengthy procedure to derive the printer calibration curves and requires an object printer/scanner and the generation of object targets and comparison data that are used to calibrate the subject printer/scanner systems. In addition, although this patent makes reference to "calibration curves", it does not provide any means or procedure to define what the optimal curves should be. For the dot gain and maximum density parameters to be set properly presently requires tedious trial and error processes.
U.S. Pat. No. 5,528,386 relates to a color printer calibration architecture for calibrating a printer to produce an accurate printer response based on a given ideal input image. A set of color patches are created, which include determined linearization and black addition, by printing and measuring patches of printer colors distributed throughout the color space. The color of each patch is measured using a spectrophotometer to determine color in terms of RGB values. The measured colors are used to build a three dimensional look-up-table relating RGB defined colors to CMY defined colors. Conversions that do not include mapped and measured points may be interpolated. The look-up table compensates for the addition of black. That is, if black is added, the measured RGB for the same CMY is much darker. Accordingly, to obtain the correct RGB, a lighter combination of CMY must be used. The '386 patent thus converts from device independent values to device dependent values in converting from RGB to CMY, by providing a lighter combination of CMY in the conversion. However, this device requires that the spectral characteristics be determined for the device in order to determine how the device will react. That is, the '386 patent attempts to understand how the output device will react in order to determine how to move from an RGB color space to the CMYK color space. Accordingly, although the intent of the '386 patent is to provide a calibration utility, it does not address optimization of the CMY curves. The desired print quality and characteristics produced by the '386 patent are achieved by creation of a transformation correction table that sits between a device dependent RGB color space of the scanner and a device dependent color space of the printer.